Foundations — Diode logic gate basics

This page assumes you have seen nothing. Before we can read the parent note Diode logic gate basics, we must earn every letter and symbol it uses. We go one brick at a time; each brick sits on the previous one.

1. Voltage — the "height" of a point in a circuit

Plain words: Voltage is a number, measured in volts (symbol $\text{V}$), that we attach to every point (called a node) in a circuit. It says how much electrical "push" that point has, compared to a reference point we agree to call zero (called ground).

The picture: Think of voltage as height on a hill. Ground is sea level ($0\text{ V}$). A point at $+5\text{ V}$ is a hilltop; a point at $0\text{ V}$ is the valley floor. Water (current) always wants to roll downhill — from higher voltage to lower voltage.

Why the topic needs it: Everything in logic is "is this wire high up the hill or down at the floor?" That single question is what HIGH and LOW will mean.

2. Current, and the arrow that shows its direction

Plain words: Current is the flow of electric charge through a wire, measured in amperes. Where voltage is a height, current is the actual water moving.

The picture: On our hill, if you open a channel between a hilltop node and a valley node, water flows down the channel. The direction of conventional current is from high voltage → low voltage (downhill), shown by an arrow.

Why the topic needs it: A diode only "does" something when current tries to flow through it. To decide if a diode is ON or OFF we must ask: "does the hill slope the right way to push water through this diode?"

3. The resistor $R$ — a soft spring toward one voltage

Plain words: A resistor (symbol $R$, a zig-zag line) is a component that resists current. It doesn't block; it just makes flow harder. The key behaviour we need: a resistor connected between a node and a fixed voltage gently drags that node toward the fixed voltage whenever nothing stronger is holding it.

The picture: Imagine a weak rubber band tied from your output wire to a fixed post.

• Tie the band to the floor (ground) → it's a pull-down resistor; if left alone the wire sags to $0\text{ V}$.
• Tie the band to the ceiling ($+V_{CC}$) → it's a pull-up resistor; if left alone the wire rises to the supply.

Why the topic needs it: In the OR gate the resistor pulls the output to the floor "by default"; in the AND gate it pulls to the ceiling "by default". The diodes only override this when they conduct. Deeper detail lives in Pull-up and Pull-down Resistors.

4. The supply $V_{CC}$ — the fixed hilltop

Plain words: $V_{CC}$ is the name we give the circuit's positive power-supply voltage (e.g. $+5\text{ V}$). The subscript $CC$ is just a historical label; read it as "the +supply".

The picture: A permanent hilltop the pull-up resistor can hang from. It never moves.

Why the topic needs it: The AND gate needs a "ceiling" to pull toward. $V_{CC}$ is that ceiling.

5. The diode — the one-way valve (anode, cathode, $V_D$)

This is the star of the show, so we build it in three sub-pieces.

5a. What it looks like and what the parts are called

Plain words: A diode has two ends. The end current can flow into is the anode; the end current flows out of is the cathode. The symbol is a triangle pointing at a bar: the triangle is the anode side, the bar is the cathode. Current is allowed only in the direction the triangle points — anode → cathode. Push the other way and the valve slams shut.

5b. Forward-bias, reverse-bias, and the magic $0.7\text{ V}$

Plain words: Whether the valve opens depends on which end sits higher on the voltage-hill.

• If the anode is higher than the cathode by at least about $0.7\text{ V}$, the diode turns ON (conducts). We say it is forward-biased.
• If the anode is not higher (equal, or lower), the diode is OFF (blocks). We say it is reverse-biased.

The number $V_D\approx 0.7\text{ V}$ (for silicon) is the forward voltage drop: the small height difference the diode keeps across itself while conducting, like a fixed step it always insists on. Where this $0.7$ comes from is the story in PN Junction Diode.

The picture: When ON, a diode behaves like a tiny $0.7\text{ V}$ battery in the wire — water pours through, but it always loses a fixed $0.7\text{ V}$ of height crossing it.

Why $V_D$ and not "just zero"? Because this fixed drop is exactly why diode-logic outputs are never a clean $0$ or $5$ volts — the parent note's whole "levels degrade" section is built on this one number. If you ignore $V_D$ you get every voltage in the topic wrong by $0.7\text{ V}$.

5c. Why orientation is the logic

Plain words: Because current only flows anode→cathode, which way you point the diode chooses who can push whom.

• Anode at the input, cathode at the shared node → the input can only push the node up.
• Cathode at the input, anode at the shared node → the input can only drag the node down.

Why the topic needs it: Flip the diodes and OR becomes AND. Orientation is not decoration; it is the design decision.

6. HIGH, LOW, and logic values 0 and 1

Plain words: We agree to read voltage ranges as symbols:

• A high voltage (near $V_{CC}$) means logic 1, spoken "HIGH".
• A low voltage (near ground) means logic 0, spoken "LOW".

The picture: Two shelves on the voltage-hill: a high shelf = 1, a low shelf = 0. Anything clearly on the top shelf counts as 1; anything on the bottom shelf counts as 0. The gap between the shelves (so noise can't flip a 1 into a 0) is the subject of Logic Levels and Noise Margin.

Why the topic needs it: A "gate" is only meaningful once we can translate a messy voltage like $4.3\text{ V}$ into the clean symbol $1$.

7. $\max$, $\min$, and the "strongest voice wins" idea

Plain words: $\max (V_A,V_B)$ means "pick the larger of the two numbers"; $\min (V_A,V_B)$ means "pick the smaller". They are not new physics — just a shorthand for choosing.

The picture: Line the input voltages up on the hill. $\max$ points at the highest one; $\min$ points at the lowest one.

Why the topic needs it: The parent note's two headline formulas are $Y_{OR}=\max (V_A,V_B)-V_D,Y_{AND}=\min (V_A,V_B)+V_D.$ The $\max$/$\min$ captures "the winning input", and the $\pm V_D$ is the diode's fixed toll from step 5b. You now own every symbol in both formulas.

8. Boolean OR and AND — the truth we're aiming for

Plain words: In Boolean Algebra basics, OR outputs 1 if any input is 1; AND outputs 1 only if all inputs are 1. These are the target behaviours; the diode circuit is just a way to build them out of physics.

How the foundations feed the topic

Read it bottom-up: voltage and current let us define a diode and a resistor; orientation plus the resistor's default set the node voltage; the $0.7\text{ V}$ drop and the logic levels turn that voltage into a clean 0 or 1; $\max$/$\min$ and Boolean give us the words to describe the result.

Equipment checklist

Test yourself — you are ready for the parent note if you can answer each without peeking: